Vibrational Autoionization in Rydberg States of Ammonia

Rotationally resolved photoelectron spectroscopy is used to elucidate the mechanism for vibrational autoionization via the nontotally symmetric umbrella vibration of ammonia ( NH₃). This mechanism depends strongly on the character of the autoionizing level, and the data demonstrate that both the electronic and rotational degrees of freedom can compensate for the change in vibrational symmetry that occurs for odd changes in the quantum number of the umbrella vibration. This result has implications for models of the mode dependence of vibrational autoionization in polyatomic molecules.